Cement Slurry with Low Water to Cement Ratio

ABSTRACT

A cement slurry comprising a mixture of a cement blend and water, wherein the cement blend comprises at least about 70% by volume of blend of cementitious particulate material and water is present in the mixture in an amount of not more than 50% by volume of the slurry.

RELATED APPLICATION DATA

This application claims the benefit of EP Patent Application 06291087.2filed Jun. 29, 2006 entitled, “Cement Slurry with Low Water to CementRatio”.

TECHNICAL FIELD

This invention relates to cement slurries suitable for use inunderground wells such as oil and gas wells, and in particular relatesto slurries that can be prepared with a very low water to cement ratioyet which exhibit rapid development of compressive strength andmechanical properties of the set material.

BACKGROUND ART

In typical well cementing operations, a cement slurry is prepared at thesurface and pumped into the well through a liner or casing to fill theannulus between the casing and borehole wall to provide zonal isolationand mechanical support. The cement slurry should preferably presentrelatively low viscosity and have effectively constant rheologicalproperties while it is being prepared and pumped into the well andplaced in the zone that is to be cemented. Once it is in place, thecement will ideally develop high compressive strength in a minimum oftime. The time to develop the compressive strength is a function of thetemperature but will also depend strongly on the water to cement ratio.It is well known that extended slurries (i.e. slurries having a highwater content, typically to achieve reduced density) can take a longtime to develop sufficient compressive strength and contribute toincrease the rig time taken up in the cementing operation.

Cement slurries in widespread use for oil and gas wells typically have avolume fraction of water (volume of water/total volume of slurry,sometimes called ‘slurry porosity’) of about 59%, which corresponds to awater to cement weight ratio of about 44 wt %. It is generally acceptedthat only about 22 wt % of water is needed for the hydration of thePortland cement, the excess water in the slurry causing the developmentof porosity in the set material. While a water to cement ratio of 44 wt% can lead to a set material having a sufficiently high compressivestrength and an acceptable permeability, the same is not true in thecase when lighter slurry densities are required.

Lightweight cement slurries are typically designed using one of threetechnologies: extended slurries, foam cements and engineered particlesize systems.

In extended slurries, the slurry density is decreased by increasing thewater to cement ratio, typically up to 100 wt % to achieve a slurrydensity of 12.5 ppg (1503 kg/m³). With such a high amount of water, thedevelopment of the compressive strength is slow and the set materialexhibits a high permeability and a low compressive strength (less than1000 psi (0.69 MPa)).

In foam cements, a base slurry having a typical water to cement ratio of44 wt % is foamed with a gas (usually nitrogen). The water to cementratio is kept constant when adding the gas. In this case, the rate ofdevelopment of the compressive strength is not affected compared to thebase slurry, but the gas introduced in the material generates porosityleading to a significant increase of the permeability and decrease ofthe final compressive strength (typically 2200 psi (1.52 MPa) for a 12.5ppg (1503 kg/m³) slurry).

In engineered particle size systems, such as those described in EP0621247 A (SOFITECH NV) Oct. 26, 1994 and WO 0109056 A (SOFITECH NV)Feb. 08, 2001, the cement is blended with other particles so that thepacking volume fraction of the solids is optimized, which allowsreduction of the amount of water needed to maintain good rheologicalproperties. This technology is an improvement compared to the previousones, as the porosity of the set material remains low whatever theslurry density (which can be controlled by selecting particulatematerials of suitable density to form the slurry) and high compressivestrength can be achieved even if the water to cement ratio in suchslurries is generally not below 50 wt %.

It is an object of the invention to provide a cement slurry system thatcan be prepared with low water to cement ratios while rapidly developinga high compressive strength.

DISCLOSURE OF THE INVENTION

A first aspect of the invention provides a cement slurry comprising amixture of a cement blend and water, wherein the cement blend comprisesat least about 70% by volume of blend (bvob) of cementitious particulatematerial and water is present in the mixture in an amount of not morethan about 50% by volume of the slurry.

The cementitious materials can comprise Portland cement, slags, fumesilica, fly ash, colloidal silica and mixtures thereof.

The cement blend can comprise other particulate materials, for exampleglass microspheres such as are typically used for control of density.

The slurry preferably contains ultrafine particles having an averageparticle size of less than 1 micron, for example silica fume orcolloidal silica.

A preferred cement blend comprises Portland cement and silica fumeand/or colloidal silica in an amount of at least about 60% bvob. In aparticularly preferred slurry, the cement blend comprises Portlandcement and silica fume and/or colloidal silica in an amount ofapproximately 100% bvob.

The cement blend typically comprises particulate materials having anaverage particle size in the range of from about 1 to about 10 microns.

Water is preferably present in an amount of about 35 to about 40% byvolume of slurry. A preferred water to cementitious material ratio is inthe range of from about 20 to about 40 wt %.

In one preferred embodiment, the cement blend comprises particulatematerials in at least two discrete particle size bands. The particulatematerials can also be present in at least three discrete particle sizebands. The cementitious material can comprise two or three of theparticle size bands.

The cement slurry can further comprise additives such as dispersants,anti-freeze, water retainers, setting accelerators or retarders, foamstabilisers, or mixtures thereof.

A second aspect of the invention provides a method of cementing a well,comprising preparing a slurry according to the first aspect of theinvention and pumping the slurry into the well.

The step of preparing the slurry preferably comprises selecting solidparticulate materials for the cement blend so as to provide a specificslurry density. Alternatively, the method can comprise foaming thecement slurry in the well to adjust its density.

MODE(S) FOR CARRYING OUT THE INVENTION

Slurry compositions according to the invention are typicallycharacterised by a high solid volume fraction, and a solid blendcontaining a high amount of cementitious materials. Suitablecementitious materials comprise solids containing calcium and/or silicacapable to produce hydrates, either with or without reacting with anyother component. For example, Portland cement, mixture of Portlandcement and slags are considered as cementitious materials as well asfume silica, fly ash and colloidal silica.

A typical blend will be bimodal or trimodal (two or three distinctparticle size bands), with cementitious materials present as two orthree of these bands in order to maximize the amount of cementitiousmaterial. When a blend is mixed with water to a slurry porosity of lessthat about 50%, a minimum amount of ultra fine particles (typically withan average particle size below 1 micron) is typically required tomaintain good mixability.

Compared to the current cement slurries that have a water to cementratio above 44% weight, the compositions related to the invention canhave a water to cement ratio down to as low as about 20% weight, whilestill exhibiting rapid development of compressive strength and a veryhigh compressive strength.

The invention can provide cement slurries having a very low water/cementratio (between about 20% and 40% weight). Compared to the three knowntechnologies (foamed cements, extended slurries and engineered particlesize distribution), it provides the following benefits:

-   -   faster development of the compressive strength which is very        important specifically when the cement slurry has to set a low        temperature (deepwater wells for example);    -   the possibility to achieve very high compressive strength at low        density, even when this type of slurry is foamed, as well as a        low permeability;    -   the possibility to cover, with a single blend composition, a        broad range of slurry densities by foaming or extending it with        water, while still providing a set material with a low        permeability and high compressive strength.

The present invention provides cement slurries with a very low water tocement ratio, in a broad range of densities, leading to a significantimprovement in the development of the compressive strength.

The slurry is composed of a solid blend (that can have bimodal, trimodalor higher particle size bands) and water.

The blend of solids is prepared so that the amount of cement (orcementitious material) in the blend is more than about 70% by volume ofblend. The specific gravities of the non-cementitious particles areselected to fit with the slurry density required.

Preferred blend compositions contain Portland cement and silica fume,with a total volume fraction of these components corresponding to morethan 60% of the total solid blend.

When ultrafine particles (silica fume or colloidal silica) are presentin the blend, it can be mixed with an amount of water that is typically35 to 50% of the volume of the final slurry. The large amount of solid(between 50 and 65% by volume of slurry), combined with a large amountof cementitious material (more than 50% by volume of blend) provides avery low water to cement ratio (between 20 and 40 weight %). This canallow a rapid development of the compressive strength and a very highcompressive strength.

Another embodiment of the invention comprises a bimodal blend containinga large amount of cementitious material (>60% by volume of blend)combined with colloidal silica or fume silica in liquid form asultrafine particles.

For severe cases such as deepwater cementing where the temperature isvery low and a low density is required, one preferred approach is todesign a blend with a very high amount of cementitious material(typically between about 60 and 100% by volume of blend) and then tofoam a slurry based on this blend in order to reach the low densityrequired. Such a system presents a water to cement ratio as low as about20% and therefore develops compressive strength rapidly even at very lowtemperature.

Suitable rheological and mechanical properties are obtained by selectingthe size and the volume distribution of the particles in such a manneras to maximize the compactness of the solid mixture.

Formulations made in accordance with the invention develop a compressivestrength significantly quicker that than those of cements having thesame density. Compressive strengths are very high and porosities verylow. As a result, permeability can be smaller by several orders ofmagnitude.

Slurries according to the invention can include one or more additives ofthe following types: dispersants, antifreeze, water retainers, cementsetting accelerators or retarders, and foam stabilizers. Where suchadditives are in liquid form (either as provided or dissolved in aliquid carrier) they are considered as part of the liquid fraction.

EXAMPLE 1

The properties of two cement slurries prepared according to theinvention are compared to a conventional cement system:

Slurry A (Invention)

A mixture of powders comprising: 55% by volume of Portland Class G, 20%by volume of a mixture of Portland micro-cement and slag having a meansize of about 1.5 μm, and 25% by volume of fume silica is prepared.Water and additives (anti-foaming agent at 0.03 gallons per sack ofpowder (0.0025 l/kg), and a super-plasticizer based on polynaphthalenesulfonate at 0.4 gallons per sack of powder (0.0334 l/kg)) are mixedwith this powder so as to ensure that the volume percentage of liquid inthe slurry is 40%. A sack of powder is defined by analogy with sacks ofcement as being a sack containing 45.359 kg of mixture, in other words 1gps=0.0834 litres of additive per kg of mixture.

Slurry B (Invention)

A mixture of powders comprising 55% by volume of Portland Class G, 20%by volume of glass microspheres having a density of 380 kg/m³, and 25%by volume of fume silica is prepared. Water and additives (anti-foamingagent at 0.03 gallons per sack of powder (0.0025 l/kg), and asuper-plasticizer based on polynaphthalene sulfonate at 0.3 gallons persack of powder (0.0250 l/kg) are mixed with this powder so as to ensurethat the volume percentage of liquid in the slurry was 40%.

Slurry C (Prior Art)

Portland cement Class G cement is mixed with fresh water, 0.03 gps(0.0025 l/kg) of antifoam, and 0.04 gps (0.0033 l/kg) of dispersant sothat the density of the slurry is 15.8 ppg (1900 kg/m³) (porosity 59%)

TABLE 1 Slurry A B C Density 2140 (17.8) 1830 (15.2) 1900 (15.8) SlurryPorosity 40% 40% 59% Water to Cement ratio 23% 29% 44% CS  138 (20000) 114 (16500)  34.5 (5000)

The densities are expressed in kg/m³ (and in pounds per gallon inparentheses). CS means compressive strength after 6 days for cement setat ambient pressure and 60 C, and it is expressed in MPa (and in poundsper square inches in parentheses).

It can be seen that for the slurries A and B, compressive strengths aremuch higher than for slurry C due lower water to cement ratios.

Example 2

The properties of foamed cement slurries prepared according to theinvention are compared to conventional foamed cement systems:

Base Slurry A (Invention)

A mixture of powders comprising 55% by volume of Portland Class G, 20%by volume of glass microspheres having a density of 380 kg/m³, and 25%by volume of fume silica is prepared. Water and a super-plasticizerbased on polynaphthalene sulfonate at 0.3 gallons per sack of powder(0.0250 l/kg)) are mixed with this powder so as to ensure that thevolume percentage of liquid in the slurry was 40%.

The slurry is foamed with various quantities of foam to obtain slurrieswhose final densities are 1440 kg/m³, 1278 kg/m³ and 1150 kg/m³.

Slurry B (Prior Art)

Portland cement Class G cement is mixed with fresh water and 0.04 gps(0.0033 l/kg) of dispersant so that the density of the slurry is 15.8ppg (1900 kg/m³)(porosity 59%).

The slurry is foamed with various quantities of foam to obtain slurrieswhose final densities are 1440 kg/m³, 1278 kg/m³ and 1150 kg/m³.

TABLE 2 Base Slurry A B Slurry 40% 59% Porosity Foam 21% 30% 37% 24% 33%39% Quality Density 1440 (12) 1278 (10.6) 1150 (9.6) 1440 (12) 1278(10.6) 1150 (9.6) Water/ 29% 44% Cement ratio CS  38 (5500)  24 (3500) 19 (2700)  14 (2000)   9 (1300)   7 (1000)

The densities are expressed in kg/m³ (and in pounds per gallon inparentheses). CS means compressive strength after 10 days for cement setat ambient pressure and temperature, and it is expressed in MPa (and inpounds per square inches in parentheses).

It can be seen that for the slurries prepared in accordance with theinvention, compressive strengths are significantly higher thanconventional foamed systems for the same slurry density due a lowerwater to cement ratio.

Example 3

The properties of three cement slurries prepared according to theinvention are shown in Table 3 below. They show the wide range of slurrydensities that can be covered by the concept and the remarkablemechanical properties of the set materials.

Slurry A (Invention)

A mixture of powders comprising 35% by volume of Portland Class G, 40%by volume of glass microspheres having a density of 380 kg/m³, 25% byvolume of fume silica is prepared. Water and a super-plasticizer basedon polynaphthalene sulfonate at 0.2 gallons per bag of powder (0.0167l/kg)) are mixed with this powder so as to ensure that the volumepercentage of liquid in the slurry is 40%.

Slurry B (Invention)

A slurry similar to slurry A is prepared but without antifoam agent.This base slurry is then foamed with a quantity of foam to obtain aslurry whose final density is 970 kg/m³.

Slurry C (Invention)

A mixture of powders comprising 50% by volume of Portland Class G, 25%by volume of glass microspheres having a density of 380 kg/m³, 25% byvolume of fume silica is prepared. Water and an anti-foaming agent at0.03 gallons per sack of powder (0.0025 l/kg) are mixed with this powderso as to ensure that the volume percentage of liquid in the slurry is60%.

TABLE 3 Base Slurry A B C Base Slurry 40% 40% 60% Porosity Foam Quality 0% 35%  0% Slurry Density 1520 (12.7) 970 (8.1) 1500 (12.5)Water/Cement 40% 40% 70% ratio CS  69 (10000)  11 (1600)  17 (2500)

The densities are expressed in kg/m³ (and in pounds per gallon inparentheses). CS means compressive strength after 10 days for cement setat ambient pressure and temperature, and it is expressed in MPa (and inpounds per square inches in parentheses).

As can be seen from the preceding examples, considerable changes can bemade to the slurries according to the invention while still remainingwithin the scope of the inventive concept. Other changes than thosedescribed can also be made.

1. A cement slurry comprising a mixture of a cement blend and water,wherein the cement blend comprises at least 70% by volume of blend ofcementitious particulate material and water is present in the mixture inan amount of not more than 50% by volume of the slurry.
 2. A cementslurry as claimed in claim 1, wherein the cementitious materials areselected from the group consisting of Portland cement, slags, fumesilica, fly ash, colloidal silica and mixtures thereof.
 3. A cementslurry as claimed in claim 1, wherein the cement blend further comprisesother particulate materials.
 4. A cement slurry as claimed in claim 3,wherein the other particulate materials comprise glass microspheres. 5.A cement slurry as claimed in claim 1, wherein the slurry containsultrafine particles having an average particle size of less than 1micron.
 6. A cement slurry as claimed in claim 5, wherein the ultrafineparticles comprise silica fume or colloidal silica.
 7. A cement slurryas claimed in claim 6, wherein the cement blend comprises Portlandcement and silica fume and/or colloidal silica in an amount of at leastabout 60% by volume of blend.
 8. A cement slurry as claimed in claim 7,wherein the cement blend comprises Portland cement and silica fumeand/or colloidal silica in an amount of approximately 100% by volume ofblend.
 9. A cement slurry as claimed in claim 1, wherein the cementblend comprises particulate materials having an average particle size inthe range of from about 1 to about 10 microns.
 10. A cement slurry asclaimed in claim 1, wherein water is present in an amount of from about35 to about 40% by volume of slurry.
 11. A cement slurry as claimed inclaim 10, wherein the water to cementitious material ratio is in therange of from about 20 to about 40 wt %.
 12. A cement slurry as claimedin claim 1, wherein the cement blend comprises particulate materials inat least two discrete particle size bands.
 13. A cement slurry asclaimed in claim 12, wherein cementitious material comprises two of theparticle size bands.
 14. A cement slurry as claimed in claim 12, whereinthe cement blend comprises particulate materials in at least threediscrete particle size bands.
 15. A cement slurry as claimed in claim14, wherein cementitious material comprises two of the particle sizebands.
 16. A cement slurry as claimed in claim 14, wherein cementitiousmaterial comprises three of the particle size bands.
 17. A cement slurryas claimed in claim 1, further comprising dispersants, anti-freeze,water retainers, setting accelerators or retarders, foam stabilisers, ormixtures thereof.
 18. A method of cementing a well, comprising preparinga slurry according to claim 1 and pumping the slurry into the well. 19.A method as claimed in claim 18, wherein the step of preparing theslurry comprises selecting solid particulate materials for the cementblend so as to provide a specific slurry density.
 20. A method asclaimed in claim 18, further comprising the step of foaming the cementslurry in the well.